Well from our perspective, that was the end of the timeline. Saying that Shiro has an excuse to reset time because Yui messed him up is similar to saying that Ren didn't win the Rider War in the main timeline. Events happened to their natural conclusion in the main world and Ren won the Rider Fight, even if he still presumably died as a result. Therefore, there should be a reason to have a final winner of the Rider Fight in the Episode Final universe where we at least see Shinji or Ren fall and we see the final result of the wish. Even as an alternate universe, there should be an excuse for one to see what happens to the end without just jumping the gun and rewriting history as Shiro ultimately did. Regardless of Shiro's rewrite, there will always be a dimension where he never did it and somehow there will be a winner in the Episode Final universe, even if we never see it because Shiro jumped the gun the moment Yui killed herself (but ultimately after the final Ryuki/Ryuga fight...seriously if Shiro lost at that point then he had all excuses to rewrite time before Ryuki/Ryuga ended; by not doing so it leaves open the possibility of someone peering into said time prior to the rewrite and see how it finally resolves before Shiro decides to change things...)